A density meter, e.g. of the kind disclosed in British Pat. No. 1,175,586, is known which produces a frequency modulated electrical output signal of the form ##EQU1## where d=the density of the fluid
T=the period of the signal, and PA0 T.sub.o, K, and d.sub.o =calibration constants of the density meter.
In use, the said frequency modulated electrical output signal of the density meter is transmitted to a density converter. The latter converts the said output signal into an analog current which is linearly related to the density of the fluid passing through the density meter, the analog current being used to actuate a visual display indicative of the density. Alternatively, the density converter may produce a direct digital output or display of density.
In such an arrangement, the density converter has to be adjusted of, if computerised, has to have keyed in information as to the particular calibration constants of the density meter to which it is connected. This means that if a density meter breaks down in the field, not only is it necessary to replace the density meter, but it is also necessary to adjust the respective density converter or to key the necessary information into the latter, since no two density meters have exactly the same calibration constants.
If the density meter were to be used to measure the density of the fuel in an aircraft fuel tank, and if the density converter were to be constituted by a flight/fuel computer, then if the density meter were to break down, the flight/fuel computer would need to be adjusted and this could necessitate considerable testing of the latter before an aircraft provided therewith could be restored to an operational condition.
An object of the present invention is therefore to provide apparatus which may comprise such a density meter and which will be readily interchangeable with like apparatus without requiring the adjustment of the density converter or the keying of information into the latter.
Another object of the present invention is to provide apparatus suitable both in construction and in weight for use within, or extending through the wall of, an aircraft fuel tank for measuring the density of the fuel therein.